Trailer brake control system with safety function

ABSTRACT

A tow ball ( 14 ) is mounted to a bracket ( 16 ) by means of a spacer ( 20 ) which incorporates transducers ( 24 ) to provide signals indicating forces existing between the towing vehicle and the trailer. A control system is described which is operable to apply braking to the trailer to modify the trailer behavior independently of operation of the vehicle brakes by the operator.

The present invention relates to trailer safety and in particular, tothe safety of trailers towed behind road vehicles.

In this document, the term “trailer” will be used to encompass caravans,trailers and other like arrangements. When towing a trailer on the road,particularly behind a private vehicle, it is common to use a singlepoint towing hitch, commonly known as a tow ball. This arrangementprovides safe control over the trailer when the towing vehicle isaccelerating, so that positive forces are being applied to the trailerthrough the hitch. However, when the towing vehicle stops accelerating,the trailer is no longer stabilised in this manner and can begin to swaydangerously, particularly when the towing vehicle decelerates.

Systems are known which determine lateral sway or acceleration of avehicle during cornering and then selectively apply braking to controlthat sway. An example of such a system is given in U.S. Pat. No.4,023,864 (Lang et al). Unfortunately, lateral sway is only one form ofvehicle instability and may not be a significant factor with vehiclestability when travelling in a straight line. Furthermore, it would bedesirable to detect and prevent vehicle instability before there islateral sway.

The present invention provides a trailer control system comprisingsensor means operable to measure forces between the towing vehicle andthe trailer, control means operable while the vehicle is in operationand in response to the measurements to assess the behaviour of thetrailer relative to the towing vehicle, the control means being furtheroperable to apply braking to the trailer to modify the trailer behaviourindependently of operation of the vehicle brakes by the operator wherebyat least load axially applied to the vehicle by the trailer iscontinuously assessed, and the control means thereby applies braking tosubstantially ensure that the load remains positive.

Preferably the system calculates whether or not towing forces are beingapplied to the trailer. The trailer may be operable to apply braking iftowing forces are not being applied. Braking may be applied until towingforces are restored. Braking is preferably enabled and disabled inpulsed manner until towing forces are restored.

The sensor means may provide information enabling the system to detectthe alignment of the trailer behind the towing vehicle. The system maybe operable to apply braking selectively to one side of the trailer orthe other to correct misalignment. The sensor means may provideinformation enabling the system to sense the vehicle turning. Preferablythe system refrains from applying braking to the trailer while thevehicle is turning.

The sensor means are preferably arranged to sense forces in the mountingarrangement by which the trailer hitch point is mounted on the towingvehicle. The sensor means may comprise a spacer member which, in use, isentrapped between the towing vehicle and the trailer hitch point, andtransducer means operable to detect forces within the spacer member. Thetransducer means may comprise strain gauges, which are preferably undercompression when the system is at rest. The transducer means maycomprise a plurality of transducers distributed around the spacermember. The system may further comprise accelerometer means operable tomeasure acceleration of the towing vehicle and/or trailer. Preferablythe accelerometer means measure acceleration of the towing vehicle, andderive the acceleration of the trailer from the measured accelerationand from the force measurements of the sensor means.

The system may further comprise actuator means operable to apply controlforces to a brake cable of the trailer. The actuator means may compriseat least one stepper motor. The actuator means may be operable to applycontrol forces independently to individual brake cables.

Examples of the present invention will now be described in more detail,by way of example only, and with reference to the accompanying drawings,in which:

FIG. 1 is a schematic elevation of a vehicle and trailer arrangement ofthe type with which the invention may be used;

FIG. 2 shows the tow ball of FIG. 1 in more detail;

FIG. 3 is a section through the tow ball of FIG. 2, along the line 3—3in FIG. 2;

FIG. 4 is a block schematic of the electrical circuits associated withthe tow ball arrangement of FIGS. 2 and 3;

FIG. 5 illustrates the measurements made by the apparatus; and

FIG. 6 illustrates the braking applied by the arrangement.

FIG. 1 shows a towing vehicle 10 towing a trailer 12 connected to thevehicle 10 at a tow ball 14.

FIG. 2 shows the tow ball arrangement in more detail (with the trailer12 unhitched). A mounting bracket 16 is carried by the rear of thevehicle and provides a mounting for the tow ball 14, by means of fixingbolts (not shown) centred at the lines 18 and positioned one to eitherside of the tow ball 14. A spacer member 20 is interposed between thetow ball 14 and mounting bracket 16.

The spacer member and associated components can be described in moredetail by reference also to FIG. 3. The spacer member 20 consists of arelatively thin block of material, such as aluminium, having the samegeneral outline as the,face of the tow ball 14 offered to the mountingbracket 16, which in this example is rectangular. Two apertures 22 areprovided for receiving mounting bolts extending between the tow ball 14and mounting bracket 16, which additionally serve to locate the spacermember 20. Four transducers 24 are provided in the region of the cornersof the spacer member 20. A central region of the spacer member 20carries a printed circuit board 26 and an attitude sensing arrangementto be described below.

Each transducer 24 is a strain gauge or series of strain gauges whichproject through both faces of the spacer member 20, as can be seen inFIG. 2, so that when the tow ball 14 is mounted on the bracket 16, themounting bolts may be tightened to pre-load the transducer 24 bycompression between the ball 14 and bracket 16. This pre-loading ensuresthat the transducers 24 can provide signals representing forces in theforward or rearward direction.

The transducers 24 form part of an electronic circuit arrangementillustrated in simple block diagram form in FIG. 4. The arrangement ofFIG. 4 is implemented primarily by means of a microprocessor basedcircuit 30 located on the circuit board 26 and accompanied byappropriate support circuits such as power supplies, memory and the like(not shown separately in FIG. 4). Each of the transducers 24 provides aninput to the microprocessor 30, as follows. The strain gauge of thetransducer 24 acts as a variable resistance in a Wheatstone bridgearrangement, the output of which is applied through a gate 32 to aninput of the microprocessor 30. The microprocessor 30 can, in thismanner, receive a signal indicating the force instantaneouslyexperienced by the transducer 24, which can be either in the positive ornegative sense, by virtue of the preloading. Each of the fourtransducers 24 provides an input to the microprocessor 30 in thismanner.

Two further inputs to the microprocessor 30 are from the attitudesensing arrangement 28, which incorporates an elevation sensor 34 and anazimuth sensor 36.

The microprocessor 30 provides three outputs, as follows. Two outputs 38are applied to a gate 40, illustrated schematically as a powertransistor, to control the operation of a motor 42 used to actuate atrailer brake, as will be described. Each output 38 is controllableentirely independently of the other. Thus, the microprocessor 30 canindependently control each brake of the trailer 12.

A third output from the microprocessor 30 is in the form of a display 44provided for the operator of the vehicle 10 and preferably mounted inthe cab, dashboard or other convenient location. The microprocessor 30is preferably linked with the display 44 by a remote telemetryarrangement such as a radio link.

The motors 42 are preferably stepper motors able to operate controlcables of the trailer brakes in order to apply the brakes. These arepreferably arranged in such a manner as to supplement a conventionaloverrun trailer braking system which can therefore operate in aconventional manner in addition to operation of the arrangements of theinvention.

The arrangements which have been described allow the microprocessor 30to obtain data from the four transducers 24 and from the elevation andazimuth sensors 34,36 and to control trailer brakes through the outputs38. The manner in which these facilities are used in accordance with theinvention can now be described in more detail by explaining theoperation of the system in various situations which will arise duringuse.

Trailer at Rest

When the trailer is at rest, the nose weight of the trailer will beapplying a vertical force to the tow ball 14, causing the tow ball 14 todeflect relative to the mounting bracket 16 and apply correspondingforces to the transducers 24. This results in signals to themicroprocessor 30 and these signals can, after calibration, be used tocalculate the current deflection of the tow ball 14, and thus the noseweight of the trailer. This value is stored for future use.

The microprocessor 30 can transmit information to the display 44, suchas the nose weight of the trailer on the vehicle.

Vehicle Moving Forward (Straight Line)

When the vehicle begins to move forward, the accelerometers 34,36 willsense the change. The microprocessor 30 can calculate the current speedof the vehicle 10 by repeatedly reading the sensors 34,36 and using theconventional velocity equation:

v _(n) =u _(n) +a _(n) t _(n)

where v_(n) is the current vehicle velocity, u_(n) is the velocitycalculated by a previous calculation, a_(n) is the acceleration reportedby the sensors 34,36 and t_(n) is the elapsed time between readings.

In addition to calculating the velocity in this manner, themicroprocessor 30 can monitor the load applied to the vehicle 10 by thetrailer 12, by reading the transducers 24. Consequently, two plots ofthe type shown in FIG. 5 can be notionally produced within themicroprocessor 30.

In FIG. 5, the horizontal axis indicates time elapsed. The lower plot 46indicates the velocity in the forward direction measured by the sensors34,36. The plot 48 represents the load applied to the bracket 16 by thetow ball 14. Initially, the load 48 will be increasing as the vehicle 10seeks to move the trailer, by overcoming friction and the like. In duecourse, this resistance is overcome at point A and the trailer begins toaccelerate. The load on the vehicle 10 begins to reduce and the trailercontinues to accelerate with the vehicle 10. At point B, the trailerresistance has been fully overcome. At this point, the microprocessor 30calculates and stores a reading for the resistive force exerted by thetrailer on the vehicle 10, for future reference.

While the trailer continues in motion (as determined by themicroprocessor 30 monitoring trailer velocity), the resistive forceexerted by the trailer weight measured instantaneously and repeatedly bythe microprocessor 30 is continuously compared with the initial storedvalue. The purpose of this is to determine whether the mean resistiveforce has dropped below the stored reference value. If so, thisindicates that the vehicle 10 is not applying towing forces to thetrailer 12, but the trailer 12 is either coasting behind the vehicle 10,or moving faster than the vehicle 10. In either case, the microprocessor30 calculates whether the outputs 38 are to be activated to actuate themotors 40 in order to apply the trailer brakes. This will cause thetrailer to slow down, which will result in an increase of the effectiveresistive force of the trailer perceived by the microprocessor 30. Thetrailer brakes continue to be used to slow down the trailer until theeffective resistive force has reached or passed the stored referencevalue. At that point, the trailer is again under towing forces from thevehicle, and will be under safe control. Once the effective gross weighthas exceeded the stored value, the trailer brakes are no longeractuated.

The recalculation is carried out frequently, preferably sufficientlyfrequently as to be, in effect, continuous, in comparison with the rateof change of the measured parameters.

It will be apparent that the algorithm used by the microprocessor 30 forcontrolling the activation of the brakes should incorporate hysteresisto avoid unnecessary operation and release of the brakes.

In addition, it is envisaged that the microprocessor 30 will not operatethe trailer brakes continuously for longer than a predetermined timeperiod, to ensure that trailer brakes do not become overheated, reducingbraking efficiency. Thus, the microprocessor 30 will operate and releasethe brakes in a pulsed manner until resistive force exerted by thetrailer has returned to a value at or above the stored value. The resultcan be illustrated diagrammatically as in FIG. 6. In FIG. 6, thehorizontal axis represents time. The vertical axis is used to illustratevarious parameters, as follows.

The trailer velocity is illustrated at 50 and is initially reducingbecause the brakes have been applied, as illustrated by the square wave52 of which the higher value illustrates application of brakes, and thelower value indicates release of brakes. The velocity 50 continues todecrease until the brakes are released at C to allow the brakes to cool.The brakes remain released until D, when they are reapplied. Between thetimes C and D, the trailer velocity 50 will increase. When the brakesare re-applied, the velocity 50 again begins to reduce. Thus, thearrangement can steadily reduce the trailer velocity in this pulsedmanner until an acceptably low trailer velocity is reached, causing thetrailer to become under control of the vehicle 10 by virtue of towingforces being applied.

In addition it is envisaged that the microprocessor 30 will use theresistive force exerted by the trailer to calculate the towed mass M ofthe trailer itself, by using the conventional force equation:

F=M×A

Where F is the force exerted by the trailer on vehicle 10, and A is theacceleration of vehicle 10 reported by sensors 34 and 36.

The microprocessor 30 can then transmit the calculated mass of thetrailer to the display 44 via the output 38.

Moving Forward (Cornering)

Repeated application of the brakes in the manner described above couldbe dangerous while the vehicle is negotiating a corner. Braking thetrailer in this situation could cause the trailer to step dangerouslyout of line with the vehicle 10 (moving sideways out of the corner).Cornering of the vehicle 10 can be sensed by the accelerometers 34,36and reported to the microprocessor 30. While cornering is beingreported, the microprocessor 30 ceases to attempt to align the trailerin a straight line behind the vehicle 10, allowing the trailer to be outof line by an amount appropriate to the speed and degree of cornering.

Moving Forward (Trailer Swaying)

The description above has indicated that each trailer brake can beindividually operated by the microprocessor 30. In particular, it isdesirable that the brakes on either side of the trailer can be operatedseparately, but may be operated individually or as a group.Consequently, if the microprocessor 30 determines (by reference to theoutputs from the transducers 24 and sensors 34,36) that the trailer isswaying from side-to-side behind the vehicle 10 which is travelling in astraight line, the microprocessor 30 can produce appropriate outputs 38to operate brakes on one side or the other of the trailer,intermittently, to bring the trailer back under control.

Reversing

The vehicle can be detected as reversing by reference either to theoutputs of the accelerometers 34,36, or by reference to the perceivednose weight, which will steadily decrease as the vehicle continues toreverse. When the microprocessor 30 senses that the vehicle isreversing, no braking is applied by the system to the trailer.

It will thus be apparent from the above that the system allows thebehaviour of the trailer to be monitored continuously, and modified whenappropriate, to assist in achieving safe operation at all times. Thismonitoring and control takes place independently of the operation of thebrakes by the vehicle operator, which can continue to operate in thenormal manner. In particular, it is important to note that the trailerbrakes may be applied by the system to control trailer behaviour, evenwhile vehicle braking is not being instructed by the vehicle operator.

It will be apparent that very many variations and modifications can bemade to the apparatus described above, without departing from the scopeof the present invention. In particular, many different types oftransducer and arrangements of transducer could be used, withappropriate adjustment of the algorithms for assessing trailer behaviourfrom their outputs. A microprocessor based arrangement is envisaged, inthe interests of size, cost, programming simplicity and the like, butother circuit arrangements could be used.

Whilst endeavouring in the foregoing specification to draw attention tothose features of the invention believed to be of particular importanceit should be understood that the applicant claims protection in respectof any patentable feature or combination of features hereinbeforereferred to and/or shown in the drawings whether or not particularemphasis has been placed thereon.

What is claimed is:
 1. A vehicle trailer control system comprisingsensor means operable to provide measurements of the forces between atowing vehicle and a trailer, control means operable while the towingvehicle is in operation and in response to signals received from thesensor means corresponding to the measurements to assess the behaviourof the trailer relative to the towing vehicle, the control means beingfurther operable to apply braking to the trailer to modify the trailerbehaviour independently of operation of the towing vehicle brakes by theoperator whereby at least load axially applied to the towing vehicle bythe trailer is assessed substantially continuously, and the controlmeans thereby applies braking to substantially ensure that a towingforce acts upon the trailer, wherein the system has accelerometer means,the accelerometer means providing signals to the control means fordetermining velocity and direction of travel of the towing vehicle,wherein braking is enabled and disabled in pulsed manner until a towingforce is restored.
 2. A system according to claim 1, wherein the sensormeans provide information enabling the system to detect alignment of thetrailer behind the towing vehicle.
 3. A system according to claim 1,wherein the accelerometer means measure acceleration of the towingvehicle, and the control means derives the acceleration of the trailerfrom the measured acceleration and from the measurements of the sensormeans.
 4. A system according to claim 1 further comprising actuatormeans operable to apply control forces to a brake cable of the trailer.5. A system according to claim 4, wherein the actuator means comprise atleast one stepper motor.
 6. A system according to claim 4, wherein theactuator means are operable to apply control forces independently toindividual brake cables.
 7. A vehicle trailer control system comprisingsensor means operable to provide measurements of the forces between atowing vehicle and a trailer, control means operable while the towingvehicle is in operation and in response to signals received from thesensor means corresponding to the measurements to assess the behaviourof the trailer relative to the towing vehicle, the control means beingfurther operable to apply braking to the trailer to modify the trailerbehaviour independently of operation of the towing vehicle brakes by theoperator whereby at least load axially applied to the towing vehicle bythe trailer is assessed substantially continuously, and the controlmeans thereby applies braking to substantially ensure that a towingforce acts upon the trailer, wherein the system has accelerometer means,the accelerometer means providing signals to the control means fordetermining velocity and direction of travel of the towing vehicle,wherein the vehicle trailer control system is operable to apply brakingselectively to one side of the trailer or the other to correctmisalignment.
 8. A system according to claim 7, wherein saidaccelerometer means is operable to provide information enabling thesystem to sense the towing vehicle turning.
 9. A system according toclaim 7, wherein the accelerometer means measure acceleration of thetowing vehicle, and the control means derives the acceleration of thetrailer from the measured acceleration and from the measurements of thesensor means.
 10. A system according to claim 7 further comprisingactuator means operable to apply control forces to a brake cable of thetrailer.
 11. A system according to claim 10, wherein the actuator meanscomprise at least one stepper motor.
 12. A system according to claim 10,wherein the actuator means are operable to apply control forcesindependently to individual brake cables.
 13. A vehicle trailer controlsystem comprising sensor means operable to provide measurements of theforces between a towing vehicle and a trailer, control means operablewhile the towing vehicle is in operation and in response to signalsreceived from the sensor means corresponding to the measurements toassess the behaviour of the trailer relative to the towing vehicle, thecontrol means being further operable to apply braking to the trailer tomodify the trailer behaviour independently of operation of the towingvehicle brakes by the operator whereby at least load axially applied tothe towing vehicle by the trailer is assessed substantiallycontinuously, and the control means thereby applies braking tosubstantially ensure that a towing force acts upon the trailer, whereinthe system has accelerometer means, the accelerometer means providingsignals to the control means for determining velocity and direction oftravel of the towing vehicle, wherein said accelerometer means isoperable to provide information enabling the system to sense the towingvehicle turning, and wherein the vehicle trailer control system refrainsfrom applying, braking to the trailer while the towing vehicle isturning.
 14. A system according to claim 13, wherein the trailer has ahitch point, and wherein the sensor means are arranged to sense forcesin a mounting arrangement by which the hitch point is mounted on thetowing vehicle.
 15. A system according to claim 14, wherein the sensormeans comprise transducer means, said transducer means being mounted todetect forces within a spacer member which, in use, is entrapped betweenthe towing vehicle and the hitch point.
 16. A system according to claim15, wherein the transducer means comprise strain gauges.
 17. A systemaccording to claim 16, wherein the strain gauges are under compressionwhen the vehicle trailer control system is at rest.
 18. A systemaccording to claim 15 wherein the transducer means comprise a pluralityof transducers distributed around the spacer member.
 19. A systemaccording to claim 13, wherein the accelerometer means measureacceleration of the towing vehicle, and the control means derives theacceleration of the trailer from the measured acceleration and from themeasurements of the sensor means.
 20. A system according to claim 13further comprising actuator means operable to apply control forces to abrake cable of the trailer.
 21. A system according to claim 20, whereinthe actuator means comprise at least one stepper motor.
 22. A systemaccording to claim 20, wherein the actuator means are operable to applycontrol forces independently to individual brake cables.